Jousting armour is insanely protective.

In normal combat you have to balance manouvrability and visibility with protection (can't fight if you can't see) but in a jousting contest your only opponent is coming from straight ahead, and you never have to get off your horse.

So jousting armour gave you impenetrable protection at the expense of being basically immobile, if you look up a frogmouth helmet you'll see that you can barely see anything out of them, in fact you have to lean your head forwards to see out at all.

That helmet was also usually rivited to your chest plate so you couldn't turn your head either, your arms had nearly now movement (since all you had to do was hold a lance straight).

All that meant there were no gaps or weak points in your armour which meant there was no risk in being hit somewhere unarmoured.

Some later jousting armour was barely even armour it was more like a small shed on top of a horse that you sat in rather than something you wore.

On top of all that, your opponent also wasn't trying to kill you, jousting lances are blunt, and usually designed to be weaker than the armour they're aiming at, so it was common to see lances splinter on impact (which also had the benefit of looking cool).

And you had a shield attached to your shoulder that sat in front of your chest as another layer of armour, and that's where your opponent was aiming, the shield was the easiest place to hit (being a large flat surface, as opposed to the rest of your armour being angled and curved to deflect blows) and the aim was to dehorse your opponent, so hitting the shield was the best way to do so.